Propagation of radio waves through a tunnel



Aug. 2, 1949. J. P. SHANKLIN PROPAGATION OF RADIO WAVES THROUGH A TUNNEL Filed July 31, 1946 FIG.1

FIG. 2

' JOHN P. 'SHANKLIN 3M @MJW Patented Aug, 2, 1949 UNITED STATES PATENT OFFICE PROPAGATION F RADIO WAVES THROUGH A TUNNEL John P. Shanklin, Baltimore, Md.

Application July 31, 1946, Serial-No. 687,479

1 Claim. 1 This invention relates to a method and means for propagating radio waves through constricted natural or man-made passageways, such, for eX- ample, as a railway tunnel.

Difficulties have been experienced in transmitting radio waves through railway tunnels at frequencies desired for railway communication purposes. However, while failure normally attended attempts at those frequencies, it was noted that communication over considerable distances could be established at much higher frequencies.

At frequencies below and even somewhat above the wave guide cut-off frequency of the structure from a dimensional viewpoint, the high dielectric constant and low resistivity of the material nor- .mally composing the walls of the structure cause .a great deal of the energy to be refracted into the walls and lost. Only at frequencies at which the angle of incidence of the transmitted waves is .such as to insure almost complete reflection from passage, of radio waves at frequencies below the cut-off frequency of the passageway when acting as a wave-guide.

It is still "another object of this invention to provide means affecting propagation through such a passageway without utilizing the properties of the passageway as a Wave-guide.

It is a still further object of this invention to provide means for propagating radio waves through such a passageway while preventing substantial energy absorption by the walls thereof.

Other objects and advantages of the invention will become apparent from a consideration of the following specification, when taken together with the accompanying drawing, in which:

Fig. l is a cross sectional view of a pair of parallel conductors constituting a balanced transmission line, showing a representation of the electrostatic field existing between them;

Fig. 2 is a diagrammatic view, in cross section, of a portion of a tunnel wall and of a railway communication type antenna, together with propagation means embodying the invention, and;

3 is a perspective view of a railway tunnel composed of the parallel wires [2. lie in a plane indicated by the reference charentrance,showing the termination structure of Va wave-propagation means embodying the inven- 'tion.

The objects-and advantages of the invention are attained by suspending a metallic ground plane structure below the ceiling of the tunnel to shield the transmitted wave from the tunnel wall material and mounting below, in parallelism with the ground plane, an insulated conductor which, with the ground plane structure, form a transmission line.

Referring'now more particularly to the drawing, there is shown in Fig 1 a diagram illustrating the development of a transmission line of the type contemplated by the invention. In this figure is showna representation of the cross section of two wires liland l I of a balanced transmission line.

As these wires carryequal voltages of opposite polarity aplane b of zero potential, or a virtual ground plane, exists between them. =If

wire Il is removed and asheetconductorplaced at b, an unbalanced transmission line results 'wh'ose surge impedance is one-half that of the original line. Thelines c represent'the electrostatic lines of force constituting the field about the two'conductors. spaced from the plane b by a distancee.

Each of the conductors is Fig. 2 illustrates the employment of such an unbalanced transmission line'in accordance with the invention. Suspended below the ceiling-l6 of the tunnel is a metallic ground'plane structure These wires acter b, which may be considered asthe equivalent-oftheground plane'bofFig. 1. Suspended below the ground plane structure and in parallelism with the plane I), in the location corresponding to the conductor III of Fig. 1, is a conductor H). The conductor I0 i insulated from the ground plane structure and the walls of the tunnel. The ground plane construction comprising the wires [2 simulates a sheet of conducting material and acts as a shield to prevent the absorption of the radiated wave by the material of the tunnel wall. Each of the wires l2 defines a plane with the conductor Ill, the planes so defined being angularly separated by equal angles. This particular feature is not essential but makes for economy of wire use for optimum shielding results. To obtain effective shielding the ratio d/e should be 2 or greater where d is the distance from the center to the edge of the array of wires 12, and e is the distance between the conductor l0 and the plane 12 containing the conductors l2.

pedance. necessary to supply a point of zero voltage to radius, ground plane 2| The greater the number of wires l2 employed the lower will be the attenuation.

Fig. 2 also illustrates the positional relationship between the unbalanced transmission line structure just described and a train-borne antenna [3 of a type used in railway communication service. The antenna [3 comprises a feed line [1 of the coaxial type, the inner conductor terminating in its upper portion in a quarter-wave length vertical member I5 and the outer conductor terminating in its upper portion in a ground plane of quarter-wave radius. As will be seen, the antenna l3 will move along the length of the unbalanced transmission line with the member 15 positioned directly below the conductor l0 and spaced uniformly therefrom by an amount to insure satisfactory coupling therebetween, as indicated by the construction lines 0 representing the electrostatic lines of force in the field generated during transmission or reception with antenna I3.

Communication may be established between 'two distant points in a tunnel by the use of an antenna of the type shown at I3 in Fig. 2 at both the transmitting and receiving points. However, unlessthe transmission line is terminated at each 'end in its surge impedance, standing waves will be set up on the line which will result in flutter if used with a moving train.

By terminating the transmission line at each of its ends in an antenna, communication may be scribed in Fig. 2. Here the line is terminated in Ma directional antenna which is directed toward the track approaching the tunnel. The directional antenna shown by Way of example is a 'vertically polarized rhombic antenna comprising the legs l9 and 20, the surge impedance of the antenna being made equal to that of the transmission line. The rhombic antenna incorporates a resistive termination which equals its surge im- This resistance is not shown. It is which the resistance may be connected and such point is supplied in this case by a quarter-wave A imilar termination will be employed on the other end of the transmission line.

The system just described may be employed not 13, emerging from a tunnel l6 which is provided with an unbalanced transmission line comprising the 'wires l2 and the insulated conductor 10, as deonly in tunnels but also in cuts and cayons or wherever a point of poor communication is found to be present due to the conditions which were described above. For outdoor use a two-wire balanced line may be used because of its cheaper construction. However, the radiation from the upper wire will represent lost energy. In the systems described the direction of energy transmission may be reversed at will with equal transfer of energyin either direction.

While the disclosure of the invention has been restricted to those embodiments which are now preferred, many variations of form and arrangement falling within the scope of the invention as defined by the appended claim, will occur to those skilled in the art. It should therefore be understood that the scope of the invention is not to be considered as limited to the embodiments described herein, but only by the scope of the appended claim.

What is claimed is:

Means for the propagation of radio waves along a railway tunnel, with respect to a rail mounted antenna traversing said tunnel, which comprises a conductor electrically insulated with respect to said tunnel, said conductor extending through said tunnel and spaced in coupling relation to said antenna throughout its traversal of said tunnel, a ground plane structure paralleling said conductor and positioned between said conductor and the wall of said tunnel, whereby said ground plane structure shields said waves from absorption by the walls of said tunnel, said conductor and said ground plane structure constituting an unbalanced transmission line, and a termination at each end of said line, said termination having a surge impedance equal to that of said line, said termination comprising a directional antenna located outside an entrance to said tunnel, said directional antenna being directed along the track approaching said tunnel, and means terminating said antenna, said means comprising a ground plane.

JOHN P. SHANKLIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,985,241 Daya Dec. 25, 1934 2,201,472 Browder May 21, 940 2,297,925 Usselman Oct. 6, 1942 2,407,417 Halstead. Sept. 10, 1946 

